1. Field of the Invention
This invention relates to a novel thermoplastic elastomer composition. More particularly, it relates to a thermoplastic elastomer composition obtained by blending a very-low-density ethylene copolymer, formed by copolymerizing ethylene with an .alpha.-olefin by the use of a specific catalyst, with an ethylene-.alpha.-olefin copolymer rubber, and to a thermoplastic elastomer composition obtained by partial cross-linking of the aforesaid blend.
2. Description of the Prior Art
Among polyolefin-based thermoplastic elastomer, blends of a hard segment comprising a crystalline polyolefin such as polyethylene or polypropylene and a soft segment comprising an amorphous copolymer rubber such as ethylene-propylene copolymer rubber (EPR) or ethylene-propylene-unconjugated diene copolymer rubber (EPDM), and compositions obtained by partial cross-linking of such blends are known. In addition, the method of synthesizing a hard segment and a soft segment by multistage polymerization is also known. Various grades of compositions ranging from flexible to rigid are being produced by varying the proportions of each segment.
Flexible grade compositions are attracting much attention because they are rubber-like materials which can be used in wide applications including automobile parts, hoses, wire coverings and packings. In the production of such flexible grade compositions, it is necessary to increase the proportion of the soft segment (such as EPR or EPDM) and decrease the proportion of the hard segment (such as polyethylene or polypro- pylene).
However, since soft segments such as EPR and EPDM have low tensile strength and are poor in thermal resistance, fluidity and oil resistance, flexible thermoplastic elastomers containing such a soft segment in high proportions also have the aforesaid disadvantages and cannot be used in a wide variety of applications. If the proportion of the soft segment is increased so as to solve these problems, the resulting composition will lack flexibility, show a reduction in such properties as permanent set, and fail to perform its function as a flexlible thermoplastic elastomer.
Where a flexible grade composition is synthesized by multistage polymerization, the hard segment and the soft segment must be synthesized separately, so that not only a very complicated apparatus is required, but also it is very difficult to control the properties and proportion of each segment at each polymerization stage. Moreover, a defective product may result at the time of switchover from one grade to another. Further, it is very difficult to recover the resulting polymer because it contains a large amount of rubber-like material.
As described above, many problems must be solved before a flexible thermoplastic elastomer of good quality can be produced.